Users of mobile telephony devices, such as mobile phones, expect the mobile telephone to be able to stay in “standby” mode for several days between recharges. However when a mobile telephone is in standby mode, it must still negotiate with the telecommunications infrastructure with which it is co-operating, and it must still be responsive to receiving a call. Therefore, it is not possible to de-power the telephone's receiver circuitry all of the time.
Within the GSM telecommunications system, the base station transmits a paging messages on a paging channel which mobile devices need to examine in order to determine if a call is being routed to them. The repetition rate of the paging message is determined by the base station operator, although it is constrained by the GSM standard. The mobile device can conserve battery power by shutting off its radio frequency receiver and much of its data processing architecture during the time period between transmissions of the paging message. However the receiver and the data processor within the mobile device needs to be operative for each reception of the paging message in order to determine whether the mobile device is being called: The paging message within the GSM standard is transmitted as four data-bursts within a single paging block. The inter-burst interval within a paging block is 4.615 ms. This makes it possible to de-power the radio receiver for some of the time between adjacent paging messages so as to reduce the current drawn from the battery of the mobile device, and thereby increase its standby time.
It is known that further advances in power reduction could be obtained if it was not necessary to receive every one of the data-bursts within the paging block all of the time.
U.S. Pat. No. 5,570,369 discloses a GSM receiver in which an attempt to decode paging data is made once two out of the four paging data bursts have been received. If the decode process fails, then the third paging data burst is received and the decode process is repeated. If it fails then the fourth paging data burst is received. It should be noted that in this arrangement two out of four data bursts of the paging block represents the absolute minimum amount of data that is required for the Viterbi decoder to function correctly.
EP 1389883 discloses an arrangement in which the message received during a first paging burst is compared with a known message. This can be achieved by computing the differences between the corresponding data bits of the incoming data and known data after equalisation. If the incoming data burst matches that expected of the known message, then known data from a known message is retrieved from memory, used to fill in the data that has yet to be received, and then decoded. A problem with this approach is that adding in three bursts of known data prior to decoding is almost certainly going to force the decoder to return data corresponding to the pre-known message. Thus the decoding process is over-constrained. If the first burst does not correspond to a pre-known message, then a second burst is received, and data from the first bursts are presented for decoding, with data for the as yet unreceived third and fourth burst being marked as unreliable, much as U.S. Pat. No. 5,570,369 operates.